FIELD OF THE INVENTION
This invention relates to a coating having good wear resistance, good processing and good initial conformability produced on a surface of a sliding substrate in a sliding part such as a piston ring.
As a typical coating treatment which is performed to obtain wear resistance required for a sliding part such as a piston ring to be used in an internal combustion engine, there is a hard chromium-plating treatment. However, due to high output power, high capacity and exhaust gas regulation requirements for an internal combustion engine, the properties required for a piston ring have become more severe and it is difficult to obtain satisfactory heat, wear and sticking resistance properties with the chromium-plating process of the prior art.
Surface treatments such as composite plating and flame spraying processes have been proposed to improve these problems. Recently, an ion-plating coating has been developed and is applied to an outer sliding surface such as that of a piston ring. A piston ring, in which a coating of chromium nitride is produced by reacting nitrogen and chromium in the vapor phase, has remarkably useful cost and wear resistance and sticking resistance properties. However, this coating is a ceramic coating and if the difference in hardness with respect to the material of a cylinder in which the piston ring is used is large, then a phenomenon in which production of blow-by gas becomes great, results from damage to the cylinder during aging at initial operation. As mentioned, wear of the ring itself is less. However, since wear of a cylinder occurs in some cases, it is said that a ring having satisfactory properties cannot be obtained. Accordingly, a soft coating such as a tin-plating or a composite plating such as Cr--N and Ti--N has been provided on the outermost surface of a ceramic coating. However, there are problems such as cost and poor adherence properties.
To solve the above-mentioned problems, an object of this invention is to provide a coating for a surface of a sliding part, the coating having satisfactory wear resistance and initial conformability and which is useful for a sliding part such as a piston ring. The coating enables such parts to be used under severe conditions in an internal combustion engine, in which a heating load is increased because of regulation of exhaust gas and high output power.